The invention relates to the field of aircraft ground proximity warning systems and more particularly to systems providing warnings of excessive aircraft descent rates.
In a prior art systems, as represented by the U.S. Pat. Bateman, No. 3,946,358 and the U.S. Pat. Bateman, No. 3,947,808 each of which is assigned to assignee of this application, the relationship between altitude above ground and aircraft descent rate is utilized to generate a warning when the aircraft descent rate is excessive for its altitude above the terrain. In the U.S. Pat. No. 3,946,358, for example, an excessive descent rate warning system is disclosed that utilizes a linear relationship between aircraft altitude above terrain and aircraft descent rate wherein the greater the aircraft's altitude above the terrain, the greater the rate of descent that is required to activate the warning signal. In the excessive descent rate warning system disclosed in the U.S. Pat. No. 3,947,808, at descent rates above a predetermined value, a warning is generated for a selected time of impact with the ground assuming the aircraft continues at that particular descent rate.
However, studies of the actual operation of ground proximity warning systems, such as those disclosed in the U.S. Pat. No. 3,946,358 and the U.S. Pat. No. 3,947,808, have indicated that pilot acceptance and confidence in the ground proximity warning system is a substantial factor in judging the value of the system from a safety standpoint. For example, in most of the prior art ground proximity warning systems the nature of the warning is an imperative command to perform a corrective maneuver such as "pull-up." In a ground Proximity warning system such as disclosed in the U.S. Pat. No. 3,946,358 where the warning may have resulted from a number of different factors including: negative climb after take off; insufficient terrain clearance; excessive sink rate; excessive closure rate with the terrain or a below glideslope condition, the pilots are inherently reluctant to act immediately due to the fact that they do not know exactly what has caused the warning. Thus, it has been found in a significant number of cases that the pilots are reluctant to respond to a purely imperative type command when the condition causing the warning is not immediately apparent. In addition, there are a significant number of controlled flight into terrain type aircraft accidents where the aircraft's descent rate has rapidly increased near the ground due to windshear or optical illusions that affect the pilot's judgement. A study of a number of these types of accidents indicate that timely warnings could be provided if the prior art warning envelopes such as those disclosed in the U.S. Pat. No. 3,946,358 and the U.S. Pat. No. 3,947,808 could be extended to provide warnings for descent rates close to the terrain on the order of 1,000 feet per minute. Unfortunately, extending the warning envelope to detect descent rates on the order of 1,000 feet per minute at altitudes in the neighborhood of 100 feet or less would result in a substantial increase in the number of unwanted or nuisance warnings generated by the warning system thereby reducing the pilot's confidence in the warning system.